siRNA-mediated knockdown of Pdcd4 expression causes upregulation of p21(Waf1/Cip1) expression

Pdcd4 directly binds the coding region of c-myb mRNA and suppresses its translation

Pdcd4 is a novel tumor suppressor protein that functions in the nucleus and the cytoplasm, and appears to be involved in the regulation of transcription and translation. In the cytoplasm, Pdcd4 has been implicated in the suppression of translation of mRNAs containing structured 5'-untranslated regions; however, the mechanisms that recruit Pdcd4 to specific target mRNAs and the identities of these mRNAs are mostly unknown. In this study, we have identified c-myb mRNA as the first natural translational target mRNA of Pdcd4. We have found that translational suppression of c-myb mRNA by Pdcd4 is dependent on sequences located within the c-myb-coding region. Furthermore, we have found that the N-terminal domain of Pdcd4 has an important role in targeting Pdcd4 to c-myb RNA by mediating preferential RNA binding to the Pdcd4-responsive region of c-myb mRNA. Overall, our work demonstrates for the first time that Pdcd4 is directly involved in translational suppression of a natural mRNA and provides the first evidence for a key role of the RNA-binding domain in targeting Pdcd4 to a specific mRNA.

Structure of the tandem MA-3 region of Pdcd4 protein and characterization of its interactions with eIF4A and eIF4G: molecular mechanisms of a tumor suppressor

One of the key regulatory points of translation initiation is recruitment of the 43S preinitation complex to the 5' mRNA cap by the eIF4F complex (eIF4A, eIF4E, and eIF4G). The tumor suppressor protein Pdcd4 has been shown to inhibit cap-dependent translation by interacting tightly with the RNA helicase eIF4A via its tandem MA-3 domains. The NMR studies reported here reveal a fairly extensive and well defined interface between the two MA-3 domains in solution, which appears to be stabilized by a network of interdomain salt bridges and hydrogen bonds, and reveals a unique orientation of the two domains. Characterization of the stoichiometry of the Pdcd4-eIF4A complex suggests that under physiological conditions Pdcd4 binds to a single molecule of eIF4A, which involves contacts with both Pdcd4 MA-3 domains. We also show that contacts mediated by a conserved acidic patch on the middle MA-3 domain of Pdcd4 are essential for forming a tight complex with eIF4A in vivo, whereas the equivalent region of the C-terminal MA-3 domain appears to have no role in complex formation in vivo. The formation of a 1:1 eIF4A-Pdcd4 complex in solution is consistent with the reported presence in vivo of only one molecule of eIF4A in the eIF4F complex. Pdcd4 has also been reported to interact directly with the middle region of eIF4G, however, we were unable to obtain any evidence for even a weak, transient direct interaction.

Programmed cell death 4 (PDCD4) expression during multistep Barrett's carcinogenesis

Aim

To test the contribution of programmed cell death 4 (PDCD4) tumour suppressor gene in Barrett's carcinogenesis.

Methods

PDCD4 immunohistochemical expression was assessed in 88 biopsy samples obtained from histologically proven long-segment Barrett's mucosa (BM; 25 non-intestinal columnar metaplasia, 25 intestinal metaplasia (IM), 16 low-grade intraepithelial neoplasia (LG-IEN), 12 high-grade IEN (HG-IEN) and 10 Barrett's adenocarcinoma (BAc)). As controls, 25 additional samples of native oesophageal mucosa (N) were obtained from patients with dyspepsia. To further support the data, the expression levels of miR-21, an important PDCD4 expression regulator, in 14 N, 5 HG-IEN and 11 BAc samples were determined by quantitative real-time PCR analysis.

Results

PDCD4 immunostaining decreased progressively and significantly with the progression of the phenotypic changes occurring during Barrett's carcinogenesis (p<0.001). Normal basal squamous epithelial layers featured strong PDCD4 nuclear immunoreaction (mostly coexisting with weak–moderate cytoplasmic staining). Non-intestinal columnar metaplasia and intestinal metaplasia preserved a strong nuclear immunostaining; conversely, a significant decrease in PDCD4 nuclear expression was seen in dysplastic (LG-IEN and HG-IEN) and neoplastic lesions. Weak–moderate cytoplasmic immunostaining was evident in cases of LG-IEN, while HG-IEN and BAc samples showed weak cytoplasmic or no protein expression. As expected, miR-21 expression was significantly upregulated in HG-IEN and BAc samples, consistently with PDCD4 dysregulation.

Conclusions

These data support a significant role for PDCD4 downregulation in the progression of BM to BAc, and confirm miR-21 as a negative regulator of PDCD4 in vivo. Further efforts are needed to validate PDCD4 as a potential prognostic marker in patients with Barrett's oesophagus.

PDCD4 nuclear loss inversely correlates with miR-21 levels in colon carcinogenesis

Programmed cell death 4 (PDCD4) has recently been demonstrated to be a new tumor suppressor gene involved in colon carcinogenesis. PDCD4 immunohistochemical expression was assessed in 300 polypoid lesions of the colon mucosa (50 hyperplastic polyps [HP], 50 serrated adenomas [SA], 50 tubular adenomas with low-grade-intraepithelial neoplasia [LG-IEN], 50 tubular adenomas with high-grade-IEN [HG-IEN]), and in 50 colon adenocarcinomas (CRC). As normal controls, we considered 50 biopsy samples obtained from patients with irritable bowel syndrome (N). We further investigated PDCD4 messenger RNA (mRNA) levels by quantitative real-time polymerase chain reaction (PCR) in a different series of N, LG-IEN, HG-IEN, and CRC biopsy samples. miR-21 expression (an important PDCD4-expression regulator) was also determined by quantitative real-time PCR and in situ hybridization. Normal colocytes and HP featured strong PDCD4 nuclear immunostaining whereas a significantly lower PDCD4 nuclear expression was observed in dysplasia (low- and high-grade adenomas and SA) and invasive CRC. PDCD4 immunostaining and mRNA levels decreased significantly as the phenotypic changes occurring during colon carcinogenesis progressively increased (p < 0.001). As expected, miR-21 expression was significantly upregulated in preneoplastic/neoplastic samples, consistent with PDCD4 downregulation. These results consistently support the use of nuclear PDCD4 immunohistochemical downregulation as a novel biomarker for the diagnosis of dysplastic/neoplastic lesions in colon biopsy samples.

Programmed cell death 4 enhances chemosensitivity of ovarian cancer cells by activating death receptor pathway in vitro and in vivo

Chemoresistance is a major cause of treatment failure in ovarian cancer. Therefore, it is necessary to explore alternative therapeutic methods to overcome drug resistance for ovarian cancer treatment. We previously reported that programmed cell death 4 (PDCD4), a tumor suppressor, significantly suppresses the malignant phenotype of ovarian cancer cells and its lost or low expression in ovarian cancer is associated with unfavorable prognosis of patients. Here we show that PDCD4 improves the sensitivity of ovarian cancer cells to platinum-based chemotherapy. Overexpression of PDCD4 enhanced chemosensitivity in SKOV3 and CAOV3 cells with low levels of PDCD4, whereas knockdown of PDCD4 reduced chemosensitivity in OVCAR3 cells with high levels of PDCD4. Subsequently, the combination of enforced PDCD4 expression with cisplatin treatment significantly suppressed ovarian tumor growth in a xenograft animal model. The PDCD4 effect appears to be specific for cisplatin and carboplatin, not affecting cyclophosphamide, etoposide, or paclitaxel. Mechanistically, PDCD4 significantly increased cisplatin-induced cleavage of caspase-3 and caspase-8, but had only a slight impact on caspase-9 cleavage and the expression of Bax and Bcl-2 in vitro and in vivo. A specific caspase-8 inhibitor, Z-ITED-FMK, attenuated cisplatin-induced apoptosis in PDCD4-overexpressing ovarian cancer cells. Taken together, our results indicate that PDCD4 enhances cisplatin-induced apoptosis by mainly activating the death receptor pathway, and PDCD4 gene transfer in combination with cisplatin therapy may break the resistance of ovarian cancer cells to chemotherapy.

Modulation of K-Ras-dependent lung tumorigenesis by MicroRNA-21

Lung cancer is the leading cause of cancer-related deaths in the world, and non-small-cell lung cancer (NSCLC) accounts for 80% of cases. MicroRNA-21 (miR-21) expression is increased and predicts poor survival in NSCLC. Although miR-21 function has been studied in vitro with cancer cell lines, the role of miR-21 in tumor development in vivo is unknown. We utilize transgenic mice with loss-of-function and gain-of-function miR-21 alleles combined with a model of NSCLC to determine the role of miR-21 in lung cancer. We show that overexpression of miR-21 enhances tumorigenesis and that genetic deletion of miR-21 partially protects against tumor formation. MiR-21 drives tumorigenesis through inhibition of negative regulators of the Ras/MEK/ERK pathway and inhibition of apoptosis.

miR-183 inhibits TGF-beta1-induced apoptosis by downregulation of PDCD4 expression in human hepatocellular carcinoma cells

Background

In recent years, some miRNAs have been reported to be connected closely with the development of human hepatocellular carcinoma. In our previous studies, a set of miRNAs were revealed to be dysregulated in HCC tissues. However, the functions of these miRNAs in HCC remain largely undefined.

Methods

The expression profiles of miR-183 were compared between HCC tissues and adjacent normal liver tissues using qRT-PCR method. This method was used to screen the potential target genes of miR-183. A luciferase reporter assay was conducted to confirm target association. Finally, the functional effect of miR-183 in hepatoma cells was examined.

Results

Among the 25 HCC samples analyzed, microRNA-183 was significantly up-regulated (twofold to 367-fold) in 17 samples compared with the matching nontumoral liver tissues. Programmed cell death 4 (PDCD4) was identified as the target gene of miR-183. Moreover, PDCD4 is a proapoptotic molecule involved in TGF-β1-induced apoptosis in human HCC cells, we found that miR-183 transfectants were resistant to apoptosis induced by TGF-β1.

Conclusions

We conclude that miR-183 can inhibit apoptosis in human HCC cells by repressing the PDCD4 expression, and miR-183 may play an important role in HCC development.

MicroRNA-21 acts as an oncomir through multiple targets in human hepatocellular carcinoma

BACKGROUND & AIMS

MicroRNA-21 negatively regulates several targets, thereby affecting tumorigenesis. However, its mechanism of action in human hepatocellular carcinoma is poorly understood, and no direct evidence has shown a correlation between microRNA-21 function and phenotype. In this study, we investigate the function of microRNA-21 as a potent oncomir and probe the relationship between microRNA-21, its targets, and phenotypic alterations.

METHODS

We designed a set of rescue experiments using different combinations of anti-microRNA-21, siRNA, and a negative control to modulate the protein level of microRNA-21 targets and resulting phenotypic alterations. MicroRNA-21 was suppressed using anti-microRNA-21 to further uncover its effect on several critical signaling pathways.

RESULTS

We demonstrate that hepatocellular carcinoma is characterized by elevated levels of microRNA-21 and marked reductions of PTEN, PDCD4, and RECK expression. Silencing of PTEN and PDCD4 to prevent their induction by anti-microRNA-21 treatment led to decreased apoptosis and increased invasion, while silencing of RECK only led to increased invasion. Moreover, knockdown of microRNA-21 resulted in alterations of the Akt signaling pathway, the expression of p21 and MMP families, which are associated with apoptosis, and the cell cycle or invasiveness of cancer cells.

CONCLUSIONS

MicroRNA-21 simultaneously regulates multiple programs that enhance cell proliferation, apoptosis or tumor invasiveness by targeting PTEN, PDCD4, and RECK in hepatocellular carcinomas. Targeting of microRNA-21 is sufficient to limit tumor cell proliferation and invasion in a manner that is likely to involve associated changes in multiple targets, suggesting that suppression of microRNA-21 may be a novel approach for the treatment of hepatocellular carcinoma.

Control of cell survival and proliferation by mammalian eukaryotic initiation factor 4B

Translation initiation plays an important role in cell growth, proliferation, and survival. The translation initiation factor eIF4B (eukaryotic initiation factor 4B) stimulates the RNA helicase activity of eIF4A in unwinding secondary structures in the 5' untranslated region (5'UTR) of the mRNA in vitro. Here, we studied the effects of eIF4B depletion in cells using RNA interference (RNAi). In agreement with the role of eIF4B in translation initiation, its depletion resulted in inhibition of this step. Selective reduction of translation was observed for mRNAs harboring strong to moderate secondary structures in their 5'UTRs. These mRNAs encode proteins, which function in cell proliferation (Cdc25C, c-myc, and ODC [ornithine decarboxylase]) and survival (Bcl-2 and XIAP [X-linked inhibitor of apoptosis]). Furthermore, eIF4B silencing led to decreased proliferation rates, promoted caspase-dependent apoptosis, and further sensitized cells to camptothecin-induced cell death. These results demonstrate that eIF4B is required for cell proliferation and survival by regulating the translation of proliferative and prosurvival mRNAs.

Downregulation of E-cadherin is an essential event in activating beta-catenin/Tcf-dependent transcription and expression of its target genes in Pdcd4 knockdown cells

We previously reported that knock-down of tumor suppressor Pdcd4 (ProgrammedCell Death 4) down-regulates E-cadherin expression and activates β-catenin/Tcf (T cell factor) dependent transcription in colon tumor cells. However, the underlying mechanism of these observations remains unknown. In this study, we demonstrated that knock-down of Pdcd4 down-regulates E-cadherin expression through elevated protein level of Snail. Over-expression of Pdcd4 up-regulates E-cadherin expression and inhibits β-catenin/Tcf dependent transcription. We then showed that knock-down of E-cadherin activates β-catenin/Tcf dependent transcription. Conversely, over-expression of E-cadherin in Pdcd4 knock-down cells inhibits β-catenin/Tcf dependent transcription. In addition, Pdcd4 knock-down stimulates u-PAR and c-Myc expression, while u-PAR and c-Myc expression can be reversed by over-expressing E-cadherin in Pdcd4 knock-down cells. Using chromatin immunoprecipitation, we demonstrated that β-catenin/Tcf4 directly binds to the promoters of u-PAR and c-myc in Pdcd4 knock-down cells. Futhermore, knock-down of u-PAR or c-Myc inhibits invasion in Pdcd4 knock-down cells, suggesting that both u-PAR and c-Myc contribute to invasion induced by Pdcd4 knock-down. Taken together, our data demonstrated that elevated Snail expression by Pdcd4 knock-down leads to down-regulation of E-cadherin resulting in activating β-catenin/Tcf dependent transcription and stimulating the expression of c-Myc and u-PAR, thus providing molecular explanation of how Pdcd4 suppresses tumor invasion.

Loss of Programmed cell death 4 (Pdcd4) associates with the progression of ovarian cancer

Background

Programmed cell death 4 (Pdcd4) is a novel tumour suppressor and originally identified as a neoplastic transformation inhibitor. The aim of this study was to investigate the expression, prognostic significance and potential function of Pdcd4 in ovarian cancer.

Results

The expression of Pdcd4 was examined in 30 normal ovarian tissues, 16 borderline and 93 malignant ovarian tissues. A continuous down regulation of Pdcd4 expression in the sequence of normal, borderline and malignant tissues was observed. The expressions of Pdcd4 in both ovarian borderline tissues and carcinomas were significantly lower than the expression in normal ovarian tissues (p < 0.001). Furthermore, patients with lower Pdcd4 expressions were found to have shorter disease-free survival (p = 0.037). The expression of Pdcd4 was also assessed by immunohistochemical analysis in 13 ovarian normal tissues and 44 carcinomas. Different subcellular localization of Pdcd4 was observed in normal compared to malignant cells. Predominant nuclear localization of Pdcd4 was found in normal ovarian tissues while ovarian carcinomas showed mainly cytoplasmic localization of Pdcd4.

Conclusion

Our study demonstrated that the loss of Pdcd4 was a common abnormality at molecular level in ovarian cancer and it might be a potential prognostic factor in ovarian cancer patients.

Disruption of the Pdcd4 tumor suppressor gene in chicken DT40 cells reveals its role in the DNA-damage response

The programmed cell death gene 4 (Pdcd4) gene has been implicated as a new tumor suppressor gene in the development of several types of human cancer. Pdcd4 interacts with the translation initiation factor, eIF4A, and is thought to act as a translation inhibitor. Here, we have used the chicken B-cell line DT40 to disrupt the Pdcd4 gene by homologous recombination. Our study shows that cells lacking a functional Pdcd4 gene are viable and have no obvious defects when cultivated under normal growth conditions. However, Pdcd4 knockout cells show an increased sensitivity to agents that cause DNA damage, such as UV light, etoposide or ethyl-methanesulfonate. In summary, our findings show that Pdcd4 has an important function in the cellular response to DNA damage. Low Pdcd4 expression, which is frequently observed in tumor cells, might therefore contribute to tumorigenesis by disturbing the cellular DNA-damage response.

The tumour suppressor Pdcd4: recent advances in the elucidation of function and regulation

Pdcd4 (programmed cell death 4) has been known as a tumour suppressor gene and potential target for anticancer therapies for several years. Initially, Pdcd4 was identified as a gene that is up-regulated during apoptosis, but its precise role still remains to be defined. However, there is increasing evidence that Pdcd4 levels influence transcription, as well as translation, modulate different signal transduction pathways and might act as a tumour suppressor. Interestingly, recent data suggest that Pdcd4 function may depend on cell type and/or genetic background. This review summarizes the current knowledge regarding the function and regulation of Pdcd4.

Differential regulation of p53 and p21 by MKRN1 E3 ligase controls cell cycle arrest and apoptosis

Makorin Ring Finger Protein 1 (MKRN1) is a transcriptional co-regulator and an E3 ligase. Here, we show that MKRN1 simultaneously functions as a differentially negative regulator of p53 and p21. In normal conditions, MKRN1 could destabilize both p53 and p21 through ubiquitination and proteasome-dependent degradation. As a result, depletion of MKRN1 induced growth arrest through activation of p53 and p21. Interestingly, MKRN1 used earlier unknown sites, K291 and K292, for p53 ubiquitination and subsequent degradation. Under severe stress conditions, however, MKRN1 primarily induced the efficient degradation of p21. This regulatory process contributed to the acceleration of DNA damage-induced apoptosis by eliminating p21. MKRN1 depletion diminished adriamycin or ultraviolet-induced cell death, whereas ectopic expression of MKRN1 facilitated apoptosis. Furthermore, MKRN1 stable cell lines that constantly produced low levels of p53 and p21 exhibited stabilization of p53, but not p21, with increased cell death on DNA damage. Our results indicate that MKRN1 exhibits dual functions of keeping cells alive by suppressing p53 under normal conditions and stimulating cell death by repressing p21 under stress conditions.

Overexpression of DNA methyltransferase 1 and its biological significance in primary hepatocellular carcinoma

AIM: To explore the relationship between DNA methyltransferase 1 (DNMT1) and hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC) and its biological significance in primary HCC.

METHODS: We carried out an immunohistochemical examination of DNMT1 in both HCC and paired non-neoplastic liver tissues from Chinese subjects. DNMT1 mRNA was further examined in HCC cell lines by real-time PCR. We inhibited DNMT1 using siRNA and detected the effect of depletion of DNMT1 on cell proliferation ability and cell apoptosis in the HCC cell line SMMC-7721.

RESULTS: DNMT1 protein expression was increased in HCCs compared to histologically normal non-neoplastic liver tissues and the incidence of DNMT1 immunoreactivity in HCCs correlated significantly with poor tumor differentiation (P = 0.014). There were more cases with DNMT1 overexpression in HCC with HBV (42.85%) than in HCC without HBV (28.57%). However, no significant difference in DNMT1 expression was found in HBV-positive and HBV-negative cases in the Chinese HCC group. There was a trend that DNMT1 RNA expression increased more in HCC cell lines than in pericarcinoma cell lines and normal liver cell lines. In addition, we inhibited DNMT1 using siRNA in the SMMC-7721 HCC cell line and found depletion of DNMT1 suppressed cells growth independent of expression of proliferating cell nuclear antigen (PCNA), even in HCC cell lines where DNMT1 was stably decreased.

CONCLUSION: The findings implied that DNMT1 plays a key role in HBV-related hepatocellular tumorigenesis. Depletion of DNMT1 mediates growth suppression in SMMC-7721 cells.

Programmed cell death protein 4 down-regulates Y-box binding protein-1 expression via a direct interaction with Twist1 to suppress cancer cell growth

Programmed cell death protein 4 (PDCD4) has recently been shown to be involved in both transcription and translation, and to regulate cell growth. However, the mechanisms underlying PDCD4 function are not well understood. In this study, we show that PDCD4 interacts directly with the transcription factor Twist1 and leads to reduced cell growth through the down-regulation of the Twist1 target gene Y-box binding protein-1 (YB-1). PDCD4 interacts with the DNA binding domain of Twist1, inhibiting its DNA binding ability and YB-1 expression. Immunohistochemical analysis showed that an inverse correlation between nuclear PDCD4 and YB-1 expression levels was observed in 37 clinical prostate cancer specimens. Growth suppression by PDCD4 expression was completely recovered by either Twist1 or YB-1 expression. Moreover, PDCD4-overexpressing cells are sensitive to cisplatin and paclitaxel but not to etoposide or 5-fluorouracil. In summary, PDCD4 negatively regulates YB-1 expression via its interaction with Twist1 and is involved in cancer cell growth and chemoresistance.